Hearing device comprising a feedback detection unit

ABSTRACT

A hearing device, e.g. a hearing aid, comprises a forward path for processing an electric signal representing sound including a) an input unit for receiving or providing an electric input signal representing sound, b) a signal processing unit, c) an output transducer for generating stimuli perceivable as sound to a user, d) a feedback detection unit configured to detect feedback or evaluate a risk of feedback via an acoustic or mechanical or electrical feedback path from said output transducer to said input unit and comprising d1) a magnitude and phase analysis unit for repeatedly determining magnitude, Mag, and phase, Phase, of said electric input signal and further parameters based thereon, and d2) a feedback conditions and detection unit configured to check criteria for magnitude and phase feedback condition, respectively, based on said values, and to provide a feedback detection signal indicative of feedback or a risk of feedback.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of copending application Ser. No.15/690,688, filed on Aug. 30, 2017, which claims priority under 35U.S.C. § 119(a) to application Ser. No. 16/186,338.6, filed in Europe onAug. 30, 2016, all of which are hereby expressly incorporated byreference into the present application.

SUMMARY

The present disclosure relates to hearing devices, e.g. hearing aids, inparticular to feedback detection and control in hearing devices.

In modern hearing aids, a feedback control system is used to minimizethe negative effects from the acoustic feedback problem.

Information about the current feedback situation is a useful knowledgeto the feedback cancellation system. A feedback detector is often usedto determine critical feedback situations, and it provides thisinformation to the feedback control system to control its behaviour, toensure the best possible feedback performance.

The present disclosure presents a method of detecting critical feedbacksituations by estimating the open loop magnitude function and the openloop phase function.

A Hearing Device:

In an aspect of the present application, a hearing device, e.g. ahearing aid, comprising a forward path for processing an electric signalrepresenting sound, the forward path comprising

-   -   an input unit for receiving or providing an electric input        signal IN representing sound,    -   a signal processing unit for applying a frequency- and/or        level-dependent gain to an input signal of the forward path and        providing a processed output signal, and    -   an output transducer for generating stimuli perceivable as sound        to a user.

The hearing device further comprises

-   -   a feedback detection unit configured to detect feedback or        evaluate a risk of feedback via an acoustic or mechanical or        electrical feedback path from said output transducer to said        input unit is provided.

A loop is defined consisting of said forward path and said feedbackpath, the loop exhibiting a loop delay D.

The feedback detection unit comprises

-   -   a magnitude and phase analysis unit for repeatedly determining        magnitude, Mag, and phase, Phase, of said electric input signal        IN, or a processed version thereof, and further configured to        determine values of loop magnitude (LpMag), loop phase        (LpPhase), loop magnitude difference (LpMagDiff), and loop phase        difference signals (LpPhaseDiff), respectively, based thereon        and on said loop delay D;    -   a feedback conditions and detection unit configured to check        criteria for magnitude and phase feedback condition,        respectively, based on said values of loop magnitude (LpMag),        loop phase (LpPhase), loop magnitude difference (LpMagDiff), and        loop phase difference signals (LpPhaseDiff), respectively, and        to provide feedback detection signal, FbDet, indicative of        feedback or a risk of feedback.

Thereby an improved feedback detection may be provided.

Acoustic feedback is taken to mean feedback that propagates as sound (inair) from an output transducer to an input transducer of the hearingdevice. Electrical feedback may e.g. be in the form of noise induced inconductors (cross-talk) or picked up from the coil of the loudspeaker.Mechanical feedback is taken to mean feedback that propagates asmechanical vibration of a housing or other physical parts of the hearingdevice from an output transducer to an input transducer as vibration.

A (feedback) loop is defined as an external (e.g. acoustic) path, whereloop delay D is taken to mean the time required for a signal to travelthrough the loop consisting of the forward path of the hearing deviceand the feedback path from output transducer to input unit of thehearing device from an output transducer to an input unit and aninternal (e.g. electric, processing) path from the input unit to theoutput transducer. The (feedback) loop defines (or exhibits) a loopdelay D.

In an embodiment, the loop consists of said forward path and theacoustic feedback path, the loop exhibiting a loop delay D_(a). In anembodiment, the loop consists of said forward path and the mechanicalfeedback path, the loop exhibiting a loop delay D_(m). In an embodiment,the loop consists of said forward path and the electric feedback path,the loop exhibiting a loop delay D_(e).

The term ‘repeatedly determining’ (magnitude, Mag, etc.) is in thepresent context taken to mean, determine according to a certain scheme,e.g. a predefined scheme, e.g. with a certain frequency, e.g. every timeframe of the input signal, or every fraction of a time frame (e.g. wherethe time to time-frequency conversion involves overlapping time frames).In an embodiment, the term ‘repeatedly determining’ is associated withfeedback loop delay D (see definition below), e.g. to ensure that valuesof the parameters in question are available (at least) with a distancein time corresponding to one or more loop delays D (i.e. to zD, where zis an integer). In an embodiment, the term ‘repeatedly determining’ istaken to include determining the parameters in question at points intime . . . m′−2D, m′−D, m′, or at points in time . . . , m′−zD, m′,where z is a positive integer, and m′ is a specific time instant of thehearing device. The term ‘repeatedly determining’ (magnitude, Mag, etc.)may include every previous successive time instant . . . , m−2, m−1, m,but may alternatively e.g. be taken mean every second (i.e. at . . .m−4, m−2, m) or every fourth (i.e. at . . . m−8, m−4, m), or everyD^(th) time instance (i.e. at . . . m−2D, m−D, m).

In an embodiment, a time unit of the hearing device is the length intime between two time instants, and in the present context of loop delayand parameters related to the loop, e.g. equal to the length in timet_(F) of a time frame (e.g. one or more ms; a time frame may e.g.comprise N_(F)=64 audio samples, which with a sampling rate f_(s) ofe.g. 20 kHz has a duration of N_(F)/f_(s), e.g. 64/20 ms=3.2 ms), or toa fraction N_(OL) thereof, e.g. a fourth (t_(F)/4) or an eighth(t_(F)/8) of time frame (e.g. in case over overlapping time frames,where a new spectrum of the electric input signal is determined everyN_(F)/N_(OL) audio sample, or every time unit TU=(N_(F)/N_(OL))/f_(s)(=0.8 ms in for N_(F)=64, N_(OL)=4, fs=20 kHz). In this example, newvalues of loop magnitude, Mag, etc., may be determined every TU=0.8 ms.In an embodiment, loop delay D is 8 ms.

In an embodiment, the magnitude and phase analysis unit is configured torepeatedly determine magnitude, Mag, and phase, Phase, of said electricinput signal IN in dependence of the feedback loop delay D, e.g. with afrequency of 1/D. An update of the loop magnitude, Mag, etc., with afrequency of 1/D as opposed to 1/TU would save processing power (andpossibly memory) in that only 1 as opposed to D/TU values (in the aboveexample D/TU=8/0.8=10) are determined (and possibly stored).

The term ‘to provide feedback detection signal, FbDet(k,m), indicativeof feedback or a risk of feedback’ is in the present context taken toinclude to provide feedback detection signal, FbDet(k,m) that indicateswhether or not a level of feedback in frequency band k at time unit m islarger than or smaller than a threshold value (a binary indication).

In an embodiment, the first and/or second threshold levels are frequencyband specific (i.e. may dependent on frequency band index k). In anembodiment, the first and/or second threshold levels can be timedependent (i.e. may depend on time index m).

In an embodiment, the hearing device comprises an analysis filter bankfor converting said electric input signal IN to a number of frequencysub-band electric input signals IN(k,m), where k and m are frequencysub-band and time indices, respectively. In an embodiment, the filterbank is used to divide a time domain input signal into time-frequencydomain (frequency sub-band) signals. For each time-frequency domainsignal, feedback detection is separately determined.

In another embodiment, however, the feedback detection is done in thetime-domain rather than time-frequency domain. In such case, thefrequency and sub-band and time indices (k,m) are not used, and insteadthe full-band time index (n) applies.

In yet another embodiment, a band-limited signal (or signal(s)) is(are)used for feedback detection. In such case band index l and the timeindex m, i.e. (l,m), can e.g. be used (where one index l′ may cover oneor more corresponding indices k (e.g. k′, k′+1, k′+2).

In the time-frequency domain, the function of the magnitude and phaseanalysis unit and the feedback conditions and detection unit,respectively may be expressed as

-   -   a magnitude and phase analysis unit for repeatedly determining        magnitude Mag(k,m) and phase Phase(k,m) of said frequency        sub-band electric input signals IN(k,m) and further configured        to determine values of loop magnitude (LpMag), loop phase        (LpPhase), loop magnitude difference (LpMagDiff), and loop phase        difference signals (LpPhaseDiff), respectively, based thereon;        and    -   a feedback conditions and detection unit configured to check        criteria for magnitude and phase feedback condition,        respectively, based on said values of loop magnitude        (LpMag(k,m)), loop phase (LpPhase(k,m)), loop magnitude        difference (LpMagDiff(k,m)), and loop phase difference signals        (LpPhaseDiff(k,m)), respectively, and to provide feedback        detection signal, FbDet(k,m), indicative of feedback or a risk        of feedback.

In an embodiment, the magnitude and phase analysis unit is configured todetermine the loop magnitude at time instant m asLpMag(k,m)=Mag(k,m)−Mag(k,m _(D)),

where Mag(k,m) is the magnitude value of the electric input signalIN(k,m) at time m, whereas Mag(k,m_(D)) denotes the magnitude of theelectric input signal IN(k,m_(D)) one feedback loop delay D earlier, and

to determine the loop phase LpPhase (in radian) at time instant m asLpPhase(k,m)=wrap(Phase(k,m)−Phase(k,m _(D))),

where wrap(.) denotes the phase wrapping operator, the loop phase thushaving a possible value range of [−π, π], and where Phase(k,m) and Phase(k,m_(D)) are the phase value of the electric input signal IN, at timeinstant m and at one feedback loop delay D earlier, respectively.

The feedback loop delay D is in the present context taken to mean thetime required for a signal to travel through the loop consisting of the(electric) forward path of the hearing device and the (acoustic)feedback path from output transducer to input unit of the haring device(as illustrated in FIG. 3). The loop delay is taken to include theprocessing delay d of the (electric) forward path of the hearing devicefrom input to output and the delay d′ of the acoustic feedback path fromthe output transducer to the input unit of the hearing device, in otherwords, loop delay D=d+d′. In an embodiment, the feedback loop delay D isassumed to be known, e.g. measured or estimated in advance of the use ofthe hearing device, and e.g. stored in a memory or otherwise built intothe system. In an embodiment, the hearing device is configured to(adaptively) measure or estimate the loop delay during use (e.g.automatically, e.g. during power-on, or initiated by a user via a userinterface, or continuously, e.g. according to a predetermined scheme orwhen certain criteria are fulfilled). In an embodiment, the hearingdevice is configured to provide one value of loop magnitude and loopphase for each time index m, or for each time period corresponding to acurrent feedback loop delay (D), i.e. at times m′=p·D, where p=0, 1, 2,. . . .

In an embodiment, the magnitude and phase analysis unit is configured todetermine the loop magnitude difference LpMagDiff(k,m) at time instant masLpMagDiff(k,m)=LpMag(k,m)−LpMag(k,m _(D)).

where LpMag(k,m) and LpMag(k,m_(D)) are the values of the loop magnitudeLpMag at time instant m and at a time instant m_(D), one feedback loopdelay D earlier, respectively, and to determine the loop phasedifference LpPhaseDiff(k,m) at time instant m asLpPhaseDiff(k,m)=wrap(LpPhase(k,m)−LpPhase(k,m _(D))).

where LpPhase(k,m) and LpPhase(k,m−D) are the values of the loop phaseLpPhase at time instant m and at a time instant m_(D) (m−D), onefeedback loop delay D earlier, respectively.

In an embodiment, the loop delay D used for calculating LpMag, LpPhase,LpMagDiff and LpPhaseDiff is a frequency dependent value of loop delayD(k), where k is the frequency sub-band index. In an embodiment, thedelay d′ of the acoustic feedback path from the output to the input ofthe hearing device is frequency dependent. In an embodiment, the delayof the hearing device itself, i.e. the processing delay d of the(electric) forward path of the hearing device from input to output isfrequency dependent. In an embodiment, the processing delay varies withfrequency. In an embodiment, the processing delay increases withfrequency (e.g. when the forward path comprises IIR filters). In anembodiment, a group delay of the acoustic feedback path is frequencydependent.

In an embodiment, the criterion for the loop magnitude feedbackcondition is defined as:LpMagDet(k,m)=min(LpMag(k,m), . . . ,LpMag(k,m _(N-D)))>MagThresh,where N is a number of loop delays, m_(N-D) is the time instant Nfeedback loop delay D earlier, and MagThresh is a loop magnitudethreshold value. In an embodiment, example values of N are 0, 1, 2, . .. . In an embodiment, the magnitude threshold value MagThresh is equalto −3 dB, or −2 dB, or −1 dB, or 0 dB, or +1 dB, or +2 dB, or +3 dB. Inan embodiment, the magnitude feedback detection signal LpMagDet is abinary signal (0 or 1).

In an embodiment, the criterion for the loop phase feedback condition isdefined as:LpPhaseDet(k,m)=abs(LpPhase(k,m))<PhaseThresh,

where PhaseThresh is a threshold value. In an embodiment, the loop phasethreshold value PhaseThresh is smaller than or equal to 0.5, 0.4, 0.3,0.2, 0.1, 0.05, or 0.01 . . . (radians). In an embodiment, the phasefeedback detection signal LpPhaseDet is a binary signal (0 or 1).

In an embodiment, a criterion for feedback detection is determined basedon a combination of the criteria for loop magnitude and loop phasefeedback conditions asFbDet(k,m)=and(LpMagDet(k,m),LpPhaseDet(k,m)).

In an embodiment, the feedback detection signal FbDet is e.g. a binarysignal (0 or 1). The expression and(crit1,crit2) is taken to mean thatfor the expression to be true criterion 1 (crit1) as well as criterion 2(crit2) have to be fulfilled.

In an embodiment, a criterion for feedback detection is determined basedon a combination of criteria for loop magnitude (LpMag) and loop phasedifference (LpPhaseDiff) feedback conditions,FbDet(k,m)=and(LpMagDet(k,m),LpPhaseDiffDet(k,m))

where a criterion for the loop phase difference feedback condition isdefined asLpPhaseDiffDet(k,m)=abs(LpPhaseDiff(k,m))<PhaseDiffThresh.

In an embodiment, the loop magnitude threshold value MagThresh is equalto −1.5 dB, and the loop phase difference threshold valuePhaseDiffThresh is equal to 0.3 (cf. e.g. FIG. 4B).

In an embodiment, the feedback detection unit further comprises a looptransfer function estimation and correction unit receiving as inputs thesignals loop magnitude LpMag and loop phase LpPhase, and provides as anoutput the complex signal LtfEst representing an estimate of the complexloop transfer function. The complex signal LtfEst comprises magnitude(LpMagEst) and phase (LpPhaseEst) of the estimated loop transferfunction. In an embodiment, the complex signal LtfEst is an outputsignal of the feedback detection unit.

In an embodiment, the loop transfer function estimation and correctionunit is configured to receive an input related to a correction of theloop transfer function, e.g. due to actions initiated in response to achange of the feedback detection signal FbDet.

In an embodiment, the loop magnitude estimate LpMagEst(k,m) is computedas the linear combination of a number P of latest values of loopmagnitude LpMag(k,m),

${{{LpMagEst}\left( {k,m} \right)} = {\sum\limits_{p = 0}^{P - 1}{\alpha_{p} \cdot {{LpMag}\left( {k,{m - p}} \right)}}}},$

where α_(p) are non-negative scaling factors, and Σα_(p)=1. Examplevalues of P and α_(p) can be P=2, and α₀=α₁=0.5. In an embodiment, P=4and α₀=0.5, α₁=0.25, α₂=0.125, α₃=0.125.

Similarly, the loop phase estimate LpPhaseEst(k,m) is in an embodimentcomputed as the linear combination of latest Q values of loop phaseLpPhase(k,m),

${{LpPhaseEst}(n)} = {\sum\limits_{q = 0}^{Q - 1}{\alpha_{q} \cdot {{{LpPhase}\left( {k,{m - q}} \right)}.}}}$

where α_(q) are non-negative scaling factors, and Σα_(q)=1.

In an embodiment, the hearing device, e.g. a hearing aid, furthercomprises an action information unit configured to take as inputs thefeedback detection signal FbDet and the loop transfer function estimateLtfEst from the feedback detection unit and to provide as an output anaction information signal ACINF. In an embodiment, the actioninformation unit ACT is configured to receive information about thecurrent (estimate) of the loop transfer function (LtfEst) AND thecurrent feedback detection (FbDet, 0 or 1). Based on this informationthe action information unit ACT decides on appropriate actions(reduction of gain, increase adaptation rate of the feedback estimation,the application of frequency shift in the forward path, frequencytransposition, notch filtering, half-wave rectification, etc.) andinitiates such actions. In an embodiment, the action information signalACINF is fed to the loop transfer function estimation and correctionunit and is configured to correct the loop transfer function. In anembodiment, such correction may be due to an action initiated inresponse to a change of the feedback detection signal FbDet. In anembodiment, such action may relate to the change of a parameter of afeedback cancellation system, to a modification of a frequency shiftapplied to a signal of the forward path, to a modification of theapplied forward gain of the forward path, etc.

In an embodiment, the action information unit ACT comprises an inputcontrol signal CTRL configured to activate actions that may influencethe feedback detection. In an embodiment, the action information unitACT is configured to receive control signals related to activation ofone or more of the following: magnitude/phase changes, application ofprobe noise, changing adaptation speed, etc.

In an embodiment, the action information unit ACT is configured to testactions activated via the control signal of the action information unitACT. In an embodiment, the feedback detection unit can be used to testthe effect of different actions, e.g. actions intended to reducefeedback, such actions being e.g. activated via the control signal ofthe action information unit ACT. The test may e.g. comprise thefollowing steps: A) the initial feedback is estimated with the feedbackdetection unit (UFFE), B) the CTRL signal to the action information unitACT imposes an action to modify feedback (e.g., Gain reduction, phasemodification, frequency transposition, compression, half-waverectification, notch filtering, etc.), C) the feedback detection isre-estimated. These two subsequent measurements are then used todetermine feedback (and the influence of the applied action).

In an embodiment, the feedback detection unit comprises differentparallel processing units for providing a feedback detection signalFbDet(D_(j)), each being configured to use a different loop delay D_(j),j=1, 2, . . . , N_(D), where N_(D) is the number of different parallelprocessing units. In an embodiment, the feedback detection unit isconfigured to apply a (e.g. logic) criterion to the feedback detectionsignals FbDet(D_(j)), j=1, 2, . . . , N_(D), to provide a resultingfeedback detection signal FbDet. In an embodiment, the resultingFbDet=OR(FbDet(D_(j))), j=1, 2, . . . , N_(D), i.e. FbDet equals to ‘1’(corresponding to feedback detection) if any (i.e. one or more) of thedifferent feedback detection signals FbDet(D_(j)) detects feedback. Inan embodiment, the criterion is that resulting feedback detection signalFbDet is equal to ‘1’, if more than one of the different feedbackdetection signals FbDet(D_(j)) detect feedback.

In an embodiment, the hearing device comprises a listening device, e.g.a hearing aid, e.g. a hearing instrument, e.g. a hearing instrumentadapted for being located at the ear or fully or partially in the earcanal of a user, e.g. a headset, an earphone, an ear protection deviceor a combination thereof. In an embodiment, the hearing device comprisesa hearing aid, a headset, an earphone, an ear protection device or acombination thereof. In an embodiment, the hearing device is orconstitutes a hearing aid.

The signal processing unit is configured for enhancing the input signalsand providing a processed output signal. In an embodiment, the hearingdevice (e.g. the signal processing unit) is adapted to provide afrequency dependent gain and/or a level dependent compression and/or atransposition (with or without frequency compression) of one or morefrequency ranges to one or more other frequency ranges, e.g. tocompensate for a hearing impairment of a user. Various aspects ofdigital hearing aids are described in [Schaub; 2008].

The hearing device comprises an output transducer adapted for providinga stimulus perceived by the user as an acoustic signal based on aprocessed electric signal. In an embodiment, the output transducercomprises a receiver (loudspeaker) for providing the stimulus as anacoustic signal to the user. In an embodiment, the output transducercomprises a vibrator for providing the stimulus as mechanical vibrationof a skull bone to the user (e.g. in a bone-attached or bone-anchoredhearing device). In general, the term ‘stimuli perceivable as sound to auser’ is taken to include acoustic stimuli (sound, e.g. from aloudspeaker), electric stimuli (e.g. from an electrode array of acochlear implant for stimulating the cochlear nerve) and mechanicalstimuli (e.g. from a vibrator of a bone conducting hearing aid).

The hearing device comprises an input transducer for providing anelectric input signal representing sound. In an embodiment, the hearingdevice comprises a directional microphone system adapted to enhance atarget acoustic source among a multitude of acoustic sources in thelocal environment of the user wearing the hearing device. In anembodiment, the directional system is adapted to detect (such asadaptively detect) from which direction a particular part of themicrophone signal originates. This can be achieved in various differentways as e.g. described in the prior art.

In an embodiment, the hearing device comprises antenna and transceivercircuitry for wirelessly receiving a direct electric input signal fromanother device, e.g. a communication device or another hearing device.

In an embodiment, the hearing device is (or comprises) a portabledevice, e.g. a device comprising a local energy source, e.g. a battery,e.g. a rechargeable battery.

The hearing device comprises a forward or signal path between an inputtransducer (microphone system and/or direct electric input (e.g. awireless receiver)) and an output transducer. The signal processing unitis located in the forward path. In an embodiment, the hearing devicecomprises an analysis path comprising functional components foranalyzing the input signal (e.g. determining a level, a modulation, atype of signal, an acoustic feedback estimate, etc.). In an embodiment,some or all signal processing of the analysis path and/or the signalpath is conducted in the frequency domain. In an embodiment, some or allsignal processing of the analysis path and/or the signal path isconducted in the time domain.

In an embodiment, an analogue electric signal representing an acousticsignal is converted to a digital audio signal in an analogue-to-digital(AD) conversion process, where the analogue signal is sampled with apredefined sampling frequency or rate f_(s), f_(s) being e.g. in therange from 8 kHz to 40 kHz (adapted to the particular needs of theapplication) to provide digital samples x_(n) (or x[n]) at discretepoints in time t_(n) (or n), each audio sample representing the value ofthe acoustic signal at t_(n) by a predefined number N_(s) of bits, N_(s)being e.g. in the range from 1 to 48 bit, e.g. 24 bits. A digital samplex has a length in time of 1/f_(s), e.g. 50 μs, for f_(s)=20 kHz. In anembodiment, a number of audio samples are arranged in a time frame. Inan embodiment, a time frame comprises 64 audio data samples. Other framelengths may be used depending on the practical application.

In an embodiment, the hearing devices comprise an analogue-to-digital(AD) converter to digitize an analogue input with a predefined samplingrate, e.g. 20 kHz. In an embodiment, the hearing devices comprise adigital-to-analogue (DA) converter to convert a digital signal to ananalogue output signal, e.g. for being presented to a user via an outputtransducer.

In an embodiment, the hearing device, e.g. the microphone unit, and orthe transceiver unit comprise(s) a TF-conversion unit for providing atime-frequency representation of an input signal. In an embodiment, thetime-frequency representation comprises an array or map of correspondingcomplex or real values of the signal in question in a particular timeand frequency range. In an embodiment, the TF conversion unit comprisesa filter bank for filtering a (time varying) input signal and providinga number of (time varying) output (sub-band) signals each comprising adistinct frequency range of the input signal. In an embodiment, the TFconversion unit comprises a Fourier transformation unit (e.g. a DFT orFFT unit) for converting a time variant input signal to a (time variant)signal in the frequency domain. In an embodiment, the frequency rangeconsidered by the hearing device from a minimum frequency f_(min) to amaximum frequency f_(max) comprises a part of the typical human audiblefrequency range from 20 Hz to 20 kHz, e.g. a part of the range from 20Hz to 12 kHz. In an embodiment, a signal of the forward and/or analysispath of the hearing device is split into a number NI of frequency bands,where NI is e.g. larger than 5, such as larger than 10, such as largerthan 50, such as larger than 100, such as larger than 500, at least someof which are processed individually. In an embodiment, the hearingdevice is/are adapted to process a signal of the forward and/or analysispath in a number NP of different frequency channels (NP≤NI). Thefrequency channels may be uniform or non-uniform in width (e.g.increasing in width with frequency), overlapping or non-overlapping.

In an embodiment, the hearing device comprises a level detector (LD) fordetermining the level of an input signal (e.g. on a band level and/or ofthe full (wide band) signal). The input level of the electric microphonesignal picked up from the user's acoustic environment is e.g. aclassifier of the environment. In an embodiment, the level detector isadapted to classify a current acoustic environment of the user accordingto a number of different (e.g. average) signal levels, e.g. as aHIGH-LEVEL or LOW-LEVEL environment.

In a particular embodiment, the hearing device comprises a voicedetector (VD) for determining whether or not an input signal comprises avoice (e.g. speech) signal (at a given point in feedback reduction unittime). A voice signal is in the present context taken to include aspeech signal from a human being. It may also include other forms ofutterances generated by the human speech system (e.g. singing). In anembodiment, the voice detector unit is adapted to classify a currentacoustic environment of the user as a VOICE or NO-VOICE environment.This has the advantage that time segments of the electric microphonesignal comprising human utterances (e.g. speech) in the user'senvironment can be identified, and thus separated from time segmentsonly comprising other sound sources (e.g. artificially generated noise).In an embodiment, the voice detector is adapted to detect as a VOICEalso the user's own voice. Alternatively, the voice detector is adaptedto exclude a user's own voice from the detection of a VOICE.

In an embodiment, the hearing device comprises an own voice detector fordetecting whether a given input sound (e.g. a voice) originates from thevoice of the user of the system. In an embodiment, the microphone systemof the hearing device is adapted to be able to differentiate between auser's own voice and another person's voice and possibly from NON-voicesounds.

In an embodiment, the hearing device comprises an acoustic (and/ormechanical and/or electrical) feedback suppression system. Acousticfeedback occurs because the output loudspeaker signal from an audiosystem providing amplification of a signal picked up by a microphone ispartly returned to the microphone via an acoustic coupling through theair or other media. The part of the loudspeaker signal returned to themicrophone is then re-amplified by the system before it is re-presentedat the loudspeaker, and again returned to the microphone. As this cyclecontinues, the effect of acoustic feedback becomes audible as artifactsor even worse, howling, when the system becomes unstable. The problemappears typically when the microphone and the loudspeaker are placedclosely together, as e.g. in hearing aids or other audio systems. Someother classic situations with feedback problem are telephony, publicaddress systems, headsets, audio conference systems, etc. Adaptivefeedback cancellation has the ability to track feedback path changesover time. It is based on a linear time invariant filter to estimate thefeedback path but its filter weights are updated over time. The filterupdate may be calculated using stochastic gradient algorithms, includingsome form of the Least Mean Square (LMS) or the Normalized LMS (NLMS)algorithms. They both have the property to minimize the error signal inthe mean square sense with the NLMS additionally normalizing the filterupdate with respect to the squared Euclidean norm of some referencesignal.

In an embodiment, the hearing device further comprises other relevantfunctionality for the application in question, e.g. compression, noisereduction, etc.

Use:

In an aspect, use of a hearing device as described above, in the‘detailed description of embodiments’ and in the claims, is moreoverprovided. In an embodiment, use is provided in a system comprising audiodistribution, e.g. a system comprising a microphone and a loudspeaker insufficiently close proximity of each other to cause feedback from theloudspeaker to the microphone during operation by a user. In anembodiment, use is provided in a system comprising one or more hearinginstruments, headsets, ear phones, active ear protection systems, etc.,e.g. in handsfree telephone systems, teleconferencing systems, publicaddress systems, karaoke systems, classroom amplification systems, etc.

A Method:

In an aspect, a method of detecting feedback in a hearing device, thehearing device comprising a forward path for processing an electricsignal representing sound is provided by the present application. Theforward path comprises

-   -   an input unit for receiving or providing an electric input        signal IN representing sound,    -   a signal processing unit for applying a frequency- and/or        level-dependent gain to an input signal of the forward path and        providing a processed output signal, and    -   an output transducer for generating stimuli perceivable as sound        to a user.

The method comprises

-   -   detecting feedback or evaluating a risk of feedback via an        acoustic or mechanical feedback path from said output transducer        to said input unit, a loop consisting of said forward path and        said acoustic or mechanical or electrical feedback path being        defined, the loop exhibiting a loop delay D; by        -   repeatedly determining magnitude Mag and phase Phase of said            (e.g. frequency sub-band) electric input signal(s) IN or a            processed version thereof;        -   determining values of loop magnitude (LpMag), loop phase            (LpPhase), loop magnitude difference (LpMagDiff), and loop            phase difference signals (LpPhaseDiff), respectively, based            thereon and on said loop delay D;        -   checking criteria for magnitude and phase feedback            condition, respectively, based on said values of loop            magnitude (LpMag), loop phase (LpPhase), loop magnitude            difference (LpMagDiff), and loop phase difference signals            (LpPhaseDiff), respectively, and        -   providing feedback detection signal, FbDet, indicative of            feedback or a risk of feedback.

It is intended that some or all of the structural features of the devicedescribed above, in the ‘detailed description of embodiments’ or in theclaims can be combined with embodiments of the method, whenappropriately substituted by a corresponding process and vice versa.Embodiments of the method have the same advantages as the correspondingdevices.

In an embodiment, the magnitude and phase analysis unit is configured torepeatedly determine magnitude, Mag, and phase, Phase, of said electricinput signal IN in dependence of the feedback loop delay D, e.g. with afrequency of 1/zD, where z is a positive integer. In an embodiment, z=1.

In an embodiment, the magnitude and phase analysis unit is configured todetermine magnitude, Mag, and phase, Phase, of said electric inputsignal IN so that values of the relevant loop parameters (magnitude andphase, etc.) are available with a distance in time of D, e.g. at m′−2D,m′−D, m′ (or more generally at m′−2zD, m′−zD, m′). In an embodiment, themagnitude and phase analysis unit is configured to determine magnitude,Mag, and phase, Phase, of said electric input signal IN so that valuesof the relevant loop parameters are available with a distance in time ofa time unit TU of the time-frequency representation of the electricinput signal IN.

In an embodiment, the loop consists of said forward path and theacoustic feedback path, the loop exhibiting a loop delay D_(a). In anembodiment, the loop consists of said forward path and the mechanicalfeedback path, the loop exhibiting a loop delay D_(m). In an embodiment,the loop consists of said forward path and the electric feedback path,the loop exhibiting a loop delay D_(e).

In an embodiment, the method comprises providing an electric inputsignal IN in a number of frequency sub-band electric input signalsIN(k,m), where k and m are frequency sub-band and time indices,respectively.

In an embodiment, the method comprises providing that the loop magnitudeand the loop phase at time instant m are determined asLpMag(k,m)=Mag(k,m)−Mag(k,m _(D)),LpPhase(k,m)=wrap(Phase(k,m)−Phase(k,m _(D))),

respectively, where Mag(k,m) and Phase(k,m) are the magnitude and phase(in radians) values, respectively, of the electric input signal IN(k,m)at time m, whereas Mag(k,m_(D)) and Phase(k,m_(D)) denotes the magnitudeand phase values, respectively, of the electric input signal IN(k,m_(D))one feedback loop delay D earlier, and where wrap(.) denotes the phasewrapping operator, the loop phase thus having a possible value range of[−π, π].

In an embodiment, the method comprises providing that the loop magnitudedifference LpMagDiff(k,m) and the loop phase difference LpPhaseDiff(k,m)at time instant m are determined asLpMagDiff(k,m)=LpMag(k,m)−LpMag(k,m _(D)).LpPhaseDiff(k,m)=wrap(LpPhase(k,m)−LpPhase(k,m _(D))).

where LpMag(k,m) and LpMag(k,m_(D)) are the values of the loop magnitudeLpMag at time instant m and at a time instant m_(D) one feedback loopdelay D earlier, respectively, and where LpPhase(k,m) andLpPhase(k,m_(D)) are the values of the loop phase LpPhase at timeinstant m and at a time instant m_(D) one feedback loop delay D earlier,respectively.

A Computer Readable Medium:

In an aspect, a tangible computer-readable medium storing a computerprogram comprising program code means for causing a data processingsystem to perform at least some (such as a majority or all) of the stepsof the method described above, in the ‘detailed description ofembodiments’ and in the claims, when said computer program is executedon the data processing system is furthermore provided by the presentapplication.

By way of example, and not limitation, such computer-readable media cancomprise RAM, ROM, EEPROM, CD-ROM or other optical disk storage,magnetic disk storage or other magnetic storage devices, or any othermedium that can be used to carry or store desired program code in theform of instructions or data structures and that can be accessed by acomputer. Disk and disc, as used herein, includes compact disc (CD),laser disc, optical disc, digital versatile disc (DVD), floppy disk andBlu-ray disc where disks usually reproduce data magnetically, whilediscs reproduce data optically with lasers. Combinations of the aboveshould also be included within the scope of computer-readable media. Inaddition to being stored on a tangible medium, the computer program canalso be transmitted via a transmission medium such as a wired orwireless link or a network, e.g. the Internet, and loaded into a dataprocessing system for being executed at a location different from thatof the tangible medium.

A Data Processing System:

In an aspect, a data processing system comprising a processor andprogram code means for causing the processor to perform at least some(such as a majority or all) of the steps of the method described above,in the ‘detailed description of embodiments’ and in the claims isfurthermore provided by the present application.

A Hearing System:

In a further aspect, a hearing system comprising a hearing device asdescribed above, in the ‘detailed description of embodiments’, and inthe claims, AND an auxiliary device is moreover provided.

In an embodiment, the system is adapted to establish a communicationlink between the hearing device and the auxiliary device to provide thatinformation (e.g. control and status signals, possibly audio signals)can be exchanged or forwarded from one to the other.

In an embodiment, the auxiliary device is or comprises an audio gatewaydevice adapted for receiving a multitude of audio signals (e.g. from anentertainment device, e.g. a TV or a music player, a telephoneapparatus, e.g. a mobile telephone or a computer, e.g. a PC) and adaptedfor selecting and/or combining an appropriate one of the received audiosignals (or combination of signals) for transmission to the hearingdevice. In an embodiment, the auxiliary device is or comprises a remotecontrol for controlling functionality and operation of the hearingdevice(s). In an embodiment, the function of a remote control isimplemented in a SmartPhone, the SmartPhone possibly running an APPallowing to control the functionality of the audio processing device viathe SmartPhone (the hearing device(s) comprising an appropriate wirelessinterface to the SmartPhone, e.g. based on Bluetooth or some otherstandardized or proprietary scheme).

In an embodiment, the auxiliary device is another hearing device. In anembodiment, the hearing system comprises two hearing devices adapted toimplement a binaural hearing system, e.g. a binaural hearing aid system.

An APP:

In a further aspect, a non-transitory application, termed an APP, isfurthermore provided by the present disclosure. The APP comprisesexecutable instructions configured to be executed on an auxiliary deviceto implement a user interface for a hearing device or a hearing systemdescribed above in the ‘detailed description of embodiments’, and in theclaims. In an embodiment, the APP is configured to run on cellularphone, e.g. a smartphone, or on another portable device allowingcommunication with said hearing device or said hearing system.

Definitions

In the present context, a ‘hearing device’ refers to a device, such ase.g. a hearing instrument or an active ear-protection device or otheraudio processing device, which is adapted to improve, augment and/orprotect the hearing capability of a user by receiving acoustic signalsfrom the user's surroundings, generating corresponding audio signals,possibly modifying the audio signals and providing the possibly modifiedaudio signals as audible signals to at least one of the user's ears. A‘hearing device’ further refers to a device such as an earphone or aheadset adapted to receive audio signals electronically, possiblymodifying the audio signals and providing the possibly modified audiosignals as audible signals to at least one of the user's ears. Suchaudible signals may e.g. be provided in the form of acoustic signalsradiated into the user's outer ears, acoustic signals transferred asmechanical vibrations to the user's inner ears through the bonestructure of the user's head and/or through parts of the middle ear aswell as electric signals transferred directly or indirectly to thecochlear nerve of the user.

The hearing device may be configured to be worn in any known way, e.g.as a unit arranged behind the ear with a tube leading radiated acousticsignals into the ear canal or with a loudspeaker arranged close to or inthe ear canal, as a unit entirely or partly arranged in the pinna and/orin the ear canal, as a unit attached to a fixture implanted into theskull bone, as an entirely or partly implanted unit, etc. The hearingdevice may comprise a single unit or several units communicatingelectronically with each other.

More generally, a hearing device comprises an input transducer forreceiving an acoustic signal from a user's surroundings and providing acorresponding input audio signal and/or a receiver for electronically(i.e. wired or wirelessly) receiving an input audio signal, a (typicallyconfigurable) signal processing circuit for processing the input audiosignal and an output means for providing an audible signal to the userin dependence on the processed audio signal. In some hearing devices, anamplifier may constitute the signal processing circuit. The signalprocessing circuit typically comprises one or more (integrated orseparate) memory elements for executing programs and/or for storingparameters used (or potentially used) in the processing and/or forstoring information relevant for the function of the hearing deviceand/or for storing information (e.g. processed information, e.g.provided by the signal processing circuit), e.g. for use in connectionwith an interface to a user and/or an interface to a programming device.In some hearing devices, the output means may comprise an outputtransducer, such as e.g. a loudspeaker for providing an air-borneacoustic signal or a vibrator for providing a structure-borne orliquid-borne acoustic signal. In some hearing devices, the output meansmay comprise one or more output electrodes for providing electricsignals.

In some hearing devices, the vibrator may be adapted to provide astructure-borne acoustic signal transcutaneously or percutaneously tothe skull bone. In some hearing devices, the vibrator may be implantedin the middle ear and/or in the inner ear. In some hearing devices, thevibrator may be adapted to provide a structure-borne acoustic signal toa middle-ear bone and/or to the cochlea. In some hearing devices, thevibrator may be adapted to provide a liquid-borne acoustic signal to thecochlear liquid, e.g. through the oval window. In some hearing devices,the output electrodes may be implanted in the cochlea or on the insideof the skull bone and may be adapted to provide the electric signals tothe hair cells of the cochlea, to one or more hearing nerves, to theauditory brainstem, to the auditory midbrain, to the auditory cortexand/or to other parts of the cerebral cortex.

A ‘hearing system’ refers to a system comprising one or two hearingdevices, and a ‘binaural hearing system’ refers to a system comprisingtwo hearing devices and being adapted to cooperatively provide audiblesignals to both of the user's ears. Hearing systems or binaural hearingsystems may further comprise one or more ‘auxiliary devices’, whichcommunicate with the hearing device(s) and affect and/or benefit fromthe function of the hearing device(s). Auxiliary devices may be e.g.remote controls, audio gateway devices, mobile phones (e.g.SmartPhones), public-address systems, car audio systems or musicplayers. Hearing devices, hearing systems or binaural hearing systemsmay e.g. be used for compensating for a hearing-impaired person's lossof hearing capability, augmenting or protecting a normal-hearingperson's hearing capability and/or conveying electronic audio signals toa person.

Embodiments of the disclosure may e.g. be useful in applications such ashearing aids, headsets, ear phones, active ear protection systems, etc.The disclosure may further be useful in applications such as handsfreetelephone systems, mobile telephones, teleconferencing systems, publicaddress systems, karaoke systems, classroom amplification systems.

BRIEF DESCRIPTION OF DRAWINGS

The aspects of the disclosure may be best understood from the followingdetailed description taken in conjunction with the accompanying figures.The figures are schematic and simplified for clarity, and they just showdetails to improve the understanding of the claims, while other detailsare left out. Throughout, the same reference numerals are used foridentical or corresponding parts. The individual features of each aspectmay each be combined with any or all features of the other aspects.These and other aspects, features and/or technical effect will beapparent from and elucidated with reference to the illustrationsdescribed hereinafter in which:

FIG. 1 shows a simplified block diagram of an embodiment of a hearingdevice comprising a feedback detector according to the presentdisclosure,

FIG. 2 shows a block diagram of a first embodiment of a feedbackdetector according to the present disclosure in a sound processingenvironment,

FIG. 3 schematically illustrates the composition of loop delay in anaudio processing device, e.g. a hearing device,

FIG. 4A shows a criterion for feedback detection determined based on acombination of the criteria for loop magnitude and loop phase feedbackconditions as FbDet(k,m)=and(LpMagDet(k,m),LpPhaseDet(k,m)), and

FIG. 4B shows a criterion for feedback detection determined based on acombination of criteria for loop magnitude (LpMag) and loop phasedifference (LpPhaseDiff) feedback conditions,FbDet(k,m)=and(LpMagDet(k,m), LpPhaseDiffDet(k,m)), and

FIG. 5 shows a simplified block diagram of an embodiment of a hearingdevice comprising a feedback detector according to the presentdisclosure when used to control various processing parts of the hearingdevice,

FIG. 6 shows an embodiment of a hearing system comprising a hearingdevice and an auxiliary device in communication with each other,

FIG. 7 shows a block diagram of a s embodiment of a feedback detectoraccording to the present disclosure in a sound processing environment,and

FIG. 8 shows a flow diagram for an embodiment of a method of detectingfeedback in a hearing device according to the present disclosure.

The figures are schematic and simplified for clarity, and they just showdetails which are essential to the understanding of the disclosure,while other details are left out. Throughout, the same reference signsare used for identical or corresponding parts.

Further scope of applicability of the present disclosure will becomeapparent from the detailed description given hereinafter. However, itshould be understood that the detailed description and specificexamples, while indicating preferred embodiments of the disclosure, aregiven by way of illustration only. Other embodiments may become apparentto those skilled in the art from the following detailed description.

DETAILED DESCRIPTION OF EMBODIMENTS

The detailed description set forth below in connection with the appendeddrawings is intended as a description of various configurations. Thedetailed description includes specific details for the purpose ofproviding a thorough understanding of various concepts. However, it willbe apparent to those skilled in the art that these concepts may bepracticed without these specific details. Several aspects of theapparatus and methods are described by various blocks, functional units,modules, components, circuits, steps, processes, algorithms, etc.(collectively referred to as “elements”). Depending upon particularapplication, design constraints or other reasons, these elements may beimplemented using electronic hardware, computer program, or anycombination thereof.

The electronic hardware may include microprocessors, microcontrollers,digital signal processors (DSPs), field programmable gate arrays(FPGAs), programmable logic devices (PLDs), gated logic, discretehardware circuits, and other suitable hardware configured to perform thevarious functionality described throughout this disclosure. Computerprogram shall be construed broadly to mean instructions, instructionsets, code, code segments, program code, programs, subprograms, softwaremodules, applications, software applications, software packages,routines, subroutines, objects, executables, threads of execution,procedures, functions, etc., whether referred to as software, firmware,middleware, microcode, hardware description language, or otherwise.

The present application relates to the field of hearing devices, e.g.hearing aids. The disclosure deals in particular with concepts forultrafast feedback estimation. The estimation is based on the amplitude,phase, and their variations over time and frequency of the forward pathof a hearing device, e.g. a hearing aid.

Time and frequency:

-   -   time index n is generally used for a (digital) time domain        signal, e.g., IN(n) means the signal IN at time index n;    -   the time/frequency domain index m is generally used as a time        frame index and k is used as a frequency index;    -   for analogue signals (before A/D-conversion), t is generally        used (e.g. IN(t)), where t denotes the continuous time.

FIG. 1 shows a simplified block diagram of an embodiment of a hearingdevice comprising a feedback detector according to the presentdisclosure. The simplified block diagram of hearing device (HD)illustrates a forward path (from input transducer (IT) to outputtransducer (OT)) for processing an input sound signal and providing aprocessed output signal. The hearing device further comprises a feedbackcancellation system FBC for estimating and cancelling (at leastdecreasing) the contribution of feedback from output to input transducerin the signal of the forward path. The feedback cancellation system FBCcomprises a feedback estimation unit FBE for estimating a currentfeedback path from output transducer to input transducer (through thefeedback path FBP) and providing a feedback estimate signal ŵ. Thefeedback cancellation system FBC further comprises a combination unit(here summation unit ‘+’) for combining the feedback estimate signal ŵwith the electric input signal y from the input transducer IT (heresubtracting ŵ from y) to provide a feedback corrected signal err, whichis fed to the signal processing unit SPU and the feedback estimationunit FBE. The hearing device HD further comprises feedback detectionunit UFFE for detecting critical feedback situations based on a signalof the forward path UIN (here tapped from signal processing unit SPU)and providing feedback detection signal UOUT. The hearing device HDfurther comprises signal processing unit for processing feedbackcorrected signal err and providing a processed signal u which is fed tothe output transducer OT for presentation to the user and to thefeedback cancellation unit FBC. The feedback detection signal UOUT maye.g. be used in the signal processing unit SPU (e.g. to control a gainin the signal processing unit) and/or in the feedback cancellation unitFBC (e.g. to control an adaptation rate of the feedback estimation unitFBE).

FIG. 2 shows a block diagram of a first embodiment of a feedbackdetector according to the present disclosure in a sound processingenvironment. The block diagram of FIG. 2 may form part of a hearingdevice receiving an electric, time-variant input signal IN(n)representing sound, where n is time, e.g. a time index. The hearingdevice comprises an analysis filter bank FBA for converting the timevariant input signal IN(n) to a number of (time-variant) sub-bandsignals IN(k,m), where k is a frequency index (k=1, . . . , K) and m isa time (frame) index. The hearing device may e.g. comprise a gain unit Gfor applying a frequency and/or level dependent gain IG (0≤IG≤IG_(max)),e.g. adapted to a user's needs, e.g. hearing impairment. The gain unitprovides a processed signal OUT(k,m) that is fed to a synthesis filterbank FBS and converted to a time variant (full-band) output signalOUT(n), which may be forwarded to another device and/or presented to oneor more users. In a hearing aid setup, the input signal IN(n) may e.g.be provided by an input unit, e.g. an input transducer IT (cf. FIG. 1).Likewise, the output signal OUT(n) may be fed to an output transducer OT(cf. FIG. 1). The signal path from the input transducer to the outputtransducer (IN(n) to OUT(n)) constitutes a forward path of the hearingdevice. The hearing device further comprises a feedback detector, UltraFast Feedback Estimator (UFFE), according to the present disclosure forproviding a feedback detection control signal UOUT and a loop transferfunction control signal LtfEst, and an action unit ACT for performing anaction based on the inputs UOUT and LtfEst from the feedback detector.The action unit ACT provides an action information signal ACINF that isfed back to the feedback detector UFFE and may be used to improve thefeedback detection. The action information signal ACINF may be used byother parts of the hearing device as well to modify conditions or modesof operation of the hearing device (indicated by its connection toprocessing unit SP, cf. dashed input to the SP unit). Actions takenbased on the signals from the feedback detector (e.g. to reducefeedback), cf. signal AC to the processing unit SP may e.g. include again reduction in the forward path (cf. signal GCtr to gain unit G),and/or the introduction or modification of a (small) frequency shift inthe forward path to de-correlate input from output, etc.

In some of the calculations performed in the feedback detector UFFE,subsequent values of parameters such as loop magnitude and loop phaseare determined at time instances in units of one loop delay. Hence,knowledge (e.g. an estimate or a measurement) of the length of one loopdelay is assumed to be available.

The loop delay is defined as the time required for the signal travellingthrough the acoustic loop, as illustrated in FIG. 3. The acoustic loopconsists of the forward path (HD), and the feedback path. The loop delayis taken to include the processing delay d of the (electric) forwardpath of the hearing device from input transducer to output transducerand the delay d′ of the acoustic feedback path from the outputtransducer to the input transducer of the hearing device, LoopDelayD=d+d′.

Typically, the acoustic part d′ of the loop delay is much less than theelectric (processing) part d of the loop delay, d′<<d. In an embodimentthe electric (processing) part d of the loop delay is in the rangebetween 2 ms and 10 ms, e.g. in the range between 5 ms and 8 ms, e.g.around 7 ms. The loop delay may be relatively constant over time (ande.g. determined in advance of operation of the hearing device) or bedifferent at different points in time, e.g. depending on the currentlyapplied algorithms in the signal processing unit (e.g. dynamicallydetermined (estimated) during use). The hearing device (HD) may e.g.comprise a memory unit wherein typical loop delays in different modes ofoperation of the hearing device are stored. In an embodiment, thehearing device is configured to measure a loop delay comprising a sum ofa delay of the forward path and a delay of the feedback path. In anembodiment, a predefined test-signal is inserted in the forward path,and its round trip travel time measured (or estimated), e.g. byidentification of the test signal when it arrives in the forward pathafter a single propagation (or a known number of propagations) of theloop.

The function of the individual units of the feedback detector (UFFE) isdescribed in following.

The Magnitude and Phase Analysis Unit M&PANA:

The magnitude and phase analysis unit M&PANA takes as an input signalfrequency sub-band signals of the forward path, here the signal IN(k,m),and comprises an accumulation and down-sampling unit and a linear to logdomain transformation unit (not shown in FIG. 2).

First, the magnitude and phase of the frequency sub-band signals aredetermined.

The outputs from the magnitude and phase analysis unit M&PANA are loopmagnitude, loop phase, loop magnitude difference, and loop phasedifference signals (i.e. signals LpMag, LpPhase, LpMagDiff, andLpPhaseDiff, respectively).

The loop magnitude (in the log domain, also termed ‘loop gain’)LpMag(k,m) at the frequency index k and time index m is computed as:LpMag(k,m)=Mag(k,m)−Mag(k,m _(D)),

where Mag(k,m) is the current magnitude value of the signal, whereasm_(D) denotes the index one feedback loop delay earlier (the correcttime frame index m_(D) can be a decimal number, and a rounding might beneeded to obtain the closest integer frame index of m_(D)). Thecorresponding output signal of the M&PANA unit is termed LpMag in FIG.2.

The loop magnitude difference LpMagDiff(k,m) is computed asLpMagDiff(k,m)=LpMag(k,m)−LpMag(k,m _(D)).

The corresponding output signal of the M&PANA unit is termed LpMagDiffin FIG. 2.

The loop phase (in radian) is computed asLpPhase(k,m)=wrap(Phase(k,m)−Phase(k,m _(D))),

where wrap(.) denotes the phase wrapping operator, the loop phase thushas a possible value range of [−π, π], and Phase(k,m) is the currentphase value of the signal. The corresponding output signal of the M&PANAunit is termed LpPhase in FIG. 2.

The loop phase difference LpPhaseDiff(k,m) is computed asLpPhaseDiff(k,m)=wrap(LpPhase(k,m)−LpPhase(k,m−D)).

The loop phase difference thus has a possible value range of [−π, π],and the corresponding output signal of the M&PANA unit is termedLgPhaseDiff in FIG. 2.

In an embodiment, several of subsequent magnitude and phase valuesand/or loop magnitude difference or loop phase difference over a periodcorresponding to the feedback loop delay are accumulated and decimatedso that there is one magnitude and phase value and/or loop magnitudedifference or loop phase difference value for each time periodcorresponding to the feedback loop delay.

The Feedback Conditions and Detection Unit FBC&D:

The feedback conditions and detection unit FBC&D takes as inputs,signals LpMag, LpPhase, LpMagDiff, and LpPhaseDiff, respectively, fromthe magnitude and phase analysis unit M&PANA. The output from thefeedback conditions and detection unit FBC&D is the feedback detectionsignal (signal FbDet or UOUT).

The feedback conditions and detection unit FBC&D is configured to checkmagnitude and phase conditions for feedback.

The magnitude feedback detection, based on the magnitude condition, isdefined as:LpMagDet(k,m)=min(LpMag(k,m), . . . ,LpMag(k,m _(N-D)))>MagThresh,

where MagThresh is a threshold value close to zero, such as −3, −2, −1,0, 1, 2, 3 . . . . The index m_(N-D) denotes the time index N feedbackloop delays earlier Example values of N can be 0, 1, 2, . . . . Themagnitude feedback detection signal LpMagDet is e.g. a binary signal (0or 1).

The phase feedback detection, based on the phase condition, is definedas:LpPhaseDet(k,m)=abs(LpPhase(k,m))<PhaseThresh,

where PhaseThresh is a threshold value close to zero, such as 0.5, 0.4,0.3, 0.2, 0.1, 0.05, 0.01 . . . . The phase feedback detection signalLpPhaseDet is e.g. a binary signal (0 or 1). Alternatively, the phasefeedback detection, based on the phase difference condition, is definedas:LpPhaseDet(k,m)=abs(LpPhaseDiff(k,m))<PhaseDiffThresh,

whereas PhaseDiffThresh is a threshold value close to zero, such as 0.5,0.4, 0.3, 0.2, 0.1, 0.05, 0.01 . . . .

In principle, for the phase feedback detection, we should ideally usethe loop phase signal LpPhase, i.e., the loop phase has to be close to0. However, due to the loop phase estimation is highly depended on thefeedback loop delay, and small deviation in loop delay can cause asignificantly biased loop phase estimate. Hence, we can use the loopphase difference LpPhaseDiff to compensate for the possible bias in loopphase estimate.

The feedback detection is then determined based on magnitude and phasefeedback detections asFbDet(k,m)=and(LpMagDet(k,m),LpPhaseDet(k,m)).

The feedback detection signal FbDet is e.g. a binary signal (0 or 1).

Two example feedback detection zones are illustrated in FIG. 4A and FIG.4B. In the FIG. 4A, the detection is based on LpMag and LpPhase, withthe respective range LpMag>0, and abs(LpPhase)<0.2. FIG. 4B illustratesan example of detection based on LpMag and LpPhaseDiff, whereLpMag>−1.5, and abs(LpPhaseDiff)<0.3.

The feedback detection signal FbDet(k,m) can be further modified byconsidering the loop gain difference signal LpMagDiff(k,m). Table 1illustrates how different values of LpMagDiff(k,m) can be interpretedand used to improve the feedback detection.

TABLE 1 The indications from the loop magnitude signal LpMagDiff(k, m).LpMagDiff(k, m) Explanation Possible Actions >0 The loop magnitude isConfirm feedback detection raising ~=0 The loop magnitude is If apreventive action was constant already applied, we expect a change inloop magnitude, this can indicate false- detection <0 The loop magnitudeis No feedback detection decreasing necessary

The Loop Transfer Function Estimation and Correction Unit EST&C:

The loop transfer function estimation and correction unit EST&C takes asinputs the signals LpMag and LpPhase, and provides the output complexsignal LtfEst comprising magnitude (LpMagEst) and phase (LpPhaseEst) ofthe estimated loop transfer function. The input ACINF is used to correctthe loop transfer function estimate, if a preventive action is appliedupon feedback detection, and this action affects the loop transferfunction.

The loop transfer function consists of two parts, the loop magnitude andphase.

The loop magnitude estimate LpMagEst(k,m) is computed as the linearcombination of latest P values of LpMag(k,m),

${{{LpMagEst}\left( {k,m} \right)} = {\sum\limits_{p = 0}^{P - 1}{\alpha_{p} \cdot {{LpMag}\left( {k,{m - p}} \right)}}}},$

where α_(p) are non-negative scaling factors, and Σα_(p)=1. Examplevalues of P and α_(p) can be P=2, and α₀=α₁=0.5. In an embodiment, P=4and α₀=0.5, α₁=0.25, α₂=0.125, α₃=0.125.

Similarly, the loop phase estimate LpPhaseEst(k,m) is computed as thelinear combination of latest Q values of LpPhase(k,m),

${{LpPhaseEst}\left( {k,m} \right)} = {\sum\limits_{q = 0}^{Q - 1}{\alpha_{q} \cdot {{{LpPhase}\left( {k,{m - q}} \right)}.}}}$

The (complex) estimate of the loop transfer function can then be writtenasLtfEst(k,m)=LpMagEst(k,m)e ^(j·LpPhaseEst(k,m))

The above expressions are stated in the time-frequency domain (indicesk,m) but may alternatively be expressed in the time domain (index n) bysubstitution.

The signal ACINF is used to correct the loop transfer function estimate,when a potential action upon feedback detection can affect the estimateitself. More details are given in the section ACT.

The function of the activation unit ACT is described in following.

The Action Information Unit ACT:

The action information unit ACT takes as inputs the feedback detectionsignal FbDet and the loop transfer function estimate LtfEst and providesas an output an action information signal ACINF.

Whenever some actions are taken due to feedback detection, this actioncan potentially affect the feedback detection itself. E.g., when thefrequency shift is applied, it modifies the loop phase, and hence thisinformation should be taken into account when detecting the phasefeedback.

Or the action information is used as part of the feedback detection,e.g., as a “gain-reduction-test” or “phase-change-test” method. In thiscase, a gain reduction or a phase change of a certain amount is appliedin the forward path upon feedback detection. In the case of a correctfeedback detection based on the loop magnitude and loop phase estimates,a reduction of loop magnitude or a phase change by the same amountshould be observed. This can be used as a confirmation of feedbackdetection.

However, in the case that a false feedback detection due to incorrectlyestimated loop magnitude and/or phase, as a consequence of, e.g.,autocorrelation in the incoming signal of the hearing device, we wouldvery likely not observe a gain reduction or a phase change, at least notby the same amount of the reduced gain or modified phase. In this case,we would declare false detection.

Generally, the action information signal can be used to improve thefeedback detection (e.g. its confidence or validity). The actual use ofthe action information signal in order to improve the feedback detectionsignal depends on the action.

The action information unit ACT is configured to receive informationabout the current (estimate) of the loop transfer function (LtfEst) ANDthe current feedback detection (FbDet, 0 or 1). Based on thisinformation the action information unit ACT decides on appropriateactions (reduction of gain, increase adaptation rate of the feedbackestimation, the application of frequency shift in the forward path, orthe like) and initiates such actions.

The action information unit ACT takes also as the input control signalCTRL, which can be used to start/stop actions independent of thefeedback detection signal FbDet.

FIG. 5 shows a simplified block diagram of an embodiment of a hearingdevice comprising a feedback detector according to the presentdisclosure when used to control various processing parts of the hearingdevice. The embodiment of hearing device of FIG. 5 is similar to theembodiment of FIG. 1. In the embodiment of FIG. 5, however, a processingpart of the forward path around the signal processing unit SPU iscarried out in frequency sub-bands. The forward path hence comprises afilter bank (analysis filter bank FBA before the signal processing unitSPU and synthesis filter bank FBS before the output transducer OT). Theforward path further comprises a configurable decorrelation unit FS inthe forward path, e.g. in the form of a frequency shift Δf (e.g. Δf<10Hz). The feedback detector is configured to provide individual controlsignals UOUT₁, UOUT₂, and UOUT₃ to the signal processing unit SPU, tothe frequency shifting unit FS and to the feedback estimation unit FBE,respectively. Based on the current feedback detection signal (and otherparameters available in the feedback detection unit UFFE), the controlsignals UOUT_(p), p=1, 2, 3 are e.g. configured to control 1) a gain (inone or more frequency sub-bands) applied by the signal processing unit,2) whether or not to apply a frequency shift (and/or an amount offrequency shift) by the decorrelation unit FS, and 3) whether or not toupdate a current feedback estimate (and/or to control an adaptation rateof an adaptive algorithm) by the feedback estimation unit (FBE).

FIG. 6 shows an embodiment of a hearing system comprising a hearingdevice and an auxiliary device in communication with each other. FIG. 6shows an embodiment of a hearing aid according to the present disclosurecomprising a BTE-part located behind an ear or a user and an ITE partlocated in an ear canal of the user.

FIG. 6 illustrates an exemplary hearing aid (HD) formed as a receiver inthe ear (RITE) type hearing aid comprising a BTE-part (BTE) adapted forbeing located behind pinna and a part (ITE) comprising an outputtransducer (e.g. a loudspeaker/receiver, SPK) adapted for being locatedin an ear canal (Ear canal) of the user (e.g. exemplifying a hearing aid(HD) as shown in FIG. 1 or 5). The BTE-part (BTE) and the ITE-part (ITE)are connected (e.g. electrically connected) by a connecting element(IC). In the embodiment of a hearing aid of FIG. 5, the BTE part (BTE)comprises two input transducers (here microphones) (IT₁, IT₂) each forproviding an electric input audio signal representative of an inputsound signal (S_(BTE)) from the environment. In the scenario of FIG. 6,the input sound signal S_(BTE) includes a contribution from sound sourceS. The hearing aid of FIG. 6 further comprises two wireless receivers(WLR₁, WLR₂) for providing respective directly received auxiliary audioand/or information signals. The hearing aid (HD) further comprises asubstrate (SUB) whereon a number of electronic components are mounted,functionally partitioned according to the application in question(analogue, digital, passive components, etc.), but including aconfigurable signal processing unit (SPU), a beam former filtering unit(BFU), and a memory unit (MEM) coupled to each other and to input andoutput transducers via electrical conductors Wx. The mentionedfunctional units (as well as other components) may be partitioned incircuits and components according to the application in question (e.g.with a view to size, power consumption, analogue vs. digital processing,etc.), e.g. integrated in one or more integrated circuits, or as acombination of one or more integrated circuits and one or more separateelectronic components (e.g. inductor, capacitor, etc.). The configurablesignal processing unit (SPU) provides an enhanced audio signal, which isintended to be presented to a user. In the embodiment of a hearing aiddevice in FIG. 6, the ITE part (ITE) comprises an output unit in theform of a loudspeaker (receiver) (SPK) for converting the electricsignal (OUT) to an acoustic signal (providing, or contributing to,acoustic signal S_(ED) at the ear drum (Ear drum). In an embodiment, theITE-part further comprises an input unit comprising an input transducer(e.g. a microphone) (IT₃) for providing an electric input audio signalrepresentative of an input sound signal S_(ITE) from the environment(including from sound source S) at or in the ear canal. In anotherembodiment, the hearing aid may comprise only the BTE-microphones(IT₁/IT₂). In another embodiment, the hearing aid may comprise only theITE-microphone (IT₃). In yet another embodiment, the hearing aid maycomprise an input unit (IT₄) located elsewhere than at the ear canal incombination with one or more input units located in the BTE-part and/orthe ITE-part. The ITE-part further comprises a guiding element, e.g. adome, (DO) for guiding and positioning the ITE-part in the ear canal ofthe user.

The hearing aid (HD) exemplified in FIG. 6 is a portable device andfurther comprises a battery (BAT) for energizing electronic componentsof the BTE- and ITE-parts.

The hearing aid (HD) may e.g. comprise a directional microphone system(beam former filtering unit (BFU)) adapted to spatially filter a targetacoustic source among a multitude of acoustic sources in the localenvironment of the user wearing the hearing aid device. In anembodiment, the directional system is adapted to detect (such asadaptively detect) from which direction a particular part of themicrophone signal (e.g. a target part and/or a noise part) originates.In an embodiment, the beam former filtering unit is adapted to receiveinputs from a user interface (e.g. a remote control or a smartphone)regarding the present target direction. The memory unit (MEM) may e.g.comprise predefined (or adaptively determined) complex, frequencydependent constants (W_(ij)) defining predefined or (or adaptivelydetermined) ‘fixed’ beam patterns (e.g. omni-directional, targetcancelling, etc.), together defining the beamformed signal Y_(BF).

The hearing aid of FIG. 6 may constitute or form part of a hearing aidand/or a binaural hearing aid system according to the presentdisclosure. The hearing aid comprises a feedback detection unit asdescribed above. The processing of an audio signal in a forward path ofthe hearing aid may e.g. be performed fully or partially in thetime-frequency domain. Likewise, the processing of signals in ananalysis or control path of the hearing aid may be fully or partiallyperformed in the time-frequency domain.

The hearing aid (HD) according to the present disclosure may comprise auser interface UI, e.g. as shown in FIG. 6 implemented in an auxiliarydevice (AUX), e.g. a remote control, e.g. implemented as an APP in asmartphone or other portable (or stationary) electronic device. In theembodiment of FIG. 6, the screen of the user interface (UI) illustratesa Feedback Detection APP, with the subtitle ‘Configure feedbackdetection. Display current feedback’ (upper part of the screen).Criteria for detecting feedback can be configured by the user via theAPP (middle part of screen denoted ‘Select feedback criteria’). Thefeedback criteria can be selected between a number of criteria, herebetween ‘Loop Magnitude’, ‘Loop Phase’ and ‘Loop Phase Difference’. Inthe screen shown in FIG. 6, criteria ‘Loop Magnitude’ and ‘Loop Phase’have been selected (as indicated by solid symbols ▪), and the user canthen set threshold values for these two criteria, increasing ordecreasing selected values by activating black arrows to the right,▴=increase, ▾=decrease). The user has selected a loop magnitudethreshold value of 0 dB, and a loop phase threshold value of 0.1 (rad).The current feedback situation determined using the selected criteria isdisplayed (lower part of screen, denoted ‘Detected feedback [0,1,]’). Avalue between 0 and 1 is used to indicate a degree of severity of thecurrent feedback (overall, although determined on a frequency sub-bandlevel). The legend is indicated as OK (

) for values below 0.5 and as critical (

) for values above 0.8. The current value of (relative) feedback isillustrated by a number (here 0.5) at a corresponding location on thehorizontal grey bar (to a value between a minimum value (Mincorresponding to 0) and a maximum value (Max corresponding to 1) with amedium value (Med corresponding to 0.5) there between). The arrows atthe bottom of the screen allow changes to a preceding and a proceedingscreen of the APP, and a tab on the circular dot between the two arrowsbrings up a menu that allows the selection of other APPs or features ofthe device. In an embodiment, the APP is configured to provide an(possibly graphic) illustration of the current feedback detection (e.g.signal FbDet(k,m)) on a frequency sub-band level, e.g. relative to acurrent feedback margin.

The auxiliary device and the hearing aid are adapted to allowcommunication of data representative of the currently selected direction(if deviating from a predetermined direction (already stored in thehearing aid)) to the hearing aid via a, e.g. wireless, communicationlink (cf. dashed arrow WL2 in FIG. 6). The communication link WL2 maye.g. be based on far field communication, e.g. Bluetooth or BluetoothLow Energy (or similar technology), implemented by appropriate antennaand transceiver circuitry in the hearing aid (HD) and the auxiliarydevice (AUX), indicated by transceiver unit WLR₂ in the hearing aid.

FIG. 7 shows a block diagram of a second embodiment of a feedbackdetector according to the present disclosure in a sound processingenvironment. The embodiment of FIG. 7 comprises the same elements as theembodiment of FIG. 2. In the embodiment of FIG. 7, the feedbackdetection unit comprises different parallel processing units (cf. lightgray shaded, overlapping units FBD_(j), j=1, 2, . . . , N_(D)) forproviding a feedback detection signal FbDet(D_(j)), and optionally asignal (LtfEst(D_(j)), j=1, 2, N_(D) representing an estimate of thecomplex loop transfer function. Each parallel processing unit isconfigured to use a different loop delay D_(j), j=1, 2, . . . , N_(D),where N_(D) is the number of different parallel processing units. Thisembodiment may be of special value in situations, where the delay (d′)of the acoustic feedback path is significant compared to the delay (d)of the electric forward path of the hearing device, so that theassumption that d′<<d (cf. FIG. 3) is no longer valid or valid to asmaller degree. Such situation may e.g. occur when a (acoustically)reflecting surface is (brought) in the vicinity of the user, e.g. lessthan 1 m from an ear of the user. In such case, a parallel estimate ofthe feedback situation for different assumed values of loop delay D_(j),is of value. In an embodiment, the feedback detection unit (UFFE) isconfigured to assign values of loop delay from a minimum value (D₁) to amaximum value (D_(ND)) according to a predefined or adaptive scheme,where D₁<D₂ . . . <D_(ND). The embodiment of a feedback detection unitshown in FIG. 7 further comprises a weighting unit (WU) configured toprovide output feedback detection signals (FbDet and LtfEst) based oninput signals from the parallel processing units FBD_(j), j=1, . . . ,N_(D)), e.g. according to a predefined or adaptively determinedcriterion (e.g. a logic, e.g. Boolean, criterion). In an embodiment, theweighting unit (WU) is configured to select one of the feedbackdetection signals (FbDet(D_(j)), and LtfEst(D_(j))) as the outputfeedback detection signals (FbDet and LtfEst). In an embodiment, theweighting unit (WU) is configured to provide a weighted combination ofthe feedback detection signals (FbDet(D_(j)) and LtfEst(D_(j))) from theindividual processing units FBD_(j), j=1, 2, . . . , N_(D). In anembodiment, the feedback detection unit is configured to apply a (e.g.logic) criterion to the feedback detection signals FbDet(D_(j)),LtfEst(D_(j)), j=1, 2, . . . , N_(D), to provide resulting feedbackdetection signal FbDet, LtfEst. In an embodiment, the resultingFbDet=AND(FbDet(D_(j))), j=1, 2, N_(D), i.e. equal to ‘1’ (correspondingto feedback detection) if any (i.e. one or more) of the differentfeedback detection signals FbDet(D) detects feedback.

In an embodiment, the criterion is that resulting feedback detectionsignal FbDet is equal to ‘1’, if more than one of the different feedbackdetection signals FbDet(D_(j)) detect feedback. In an embodiment, thefeedback detector (UFFE) has access to a measured or estimated (current)value of loop delay and is configured to give particular weight to (e.g.to select) the value(s) of the feedback detection signal (and possiblythe estimate of the complex loop transfer function) provided by theprocessing part FBD_(j) for which D_(j) comes closest to the measured orestimated value D_(est) of loop delay.

FIG. 8 shows a flow diagram for an embodiment of a method of detectingfeedback in a hearing device according to the present disclosure.

In the method of detecting feedback in a hearing device, the hearingdevice comprises a forward path for processing an electric signalrepresenting sound. The forward path comprises

-   -   an input unit for receiving or providing an electric input        signal IN representing sound,    -   a signal processing unit for applying a frequency- and/or        level-dependent gain to an input signal of the forward path and        providing a processed output signal, and    -   an output transducer for generating stimuli perceivable as sound        to a user.

The method comprises

-   -   detecting feedback or evaluating a risk of feedback via an        acoustic or mechanical or electrical feedback path from said        output transducer to said input unit; by        -   repeatedly determining magnitude (Mag) and phase (Phase) of            said frequency sub-band electric input signals IN; and        -   determining values of loop magnitude (LpMag), loop phase            (LpPhase), loop magnitude difference (LpMagDiff), and loop            phase difference signals (LpPhaseDiff), respectively, based            thereon;        -   checking criteria for magnitude and phase feedback            condition, respectively, based on said values of loop            magnitude (LpMag), loop phase (LpPhase), loop magnitude            difference (LpMagDiff), and loop phase difference signals            (LpPhaseDiff), respectively, and        -   providing feedback detection signal, FbDet, indicative of            feedback or a risk of feedback.

It is intended that the structural features of the devices describedabove, either in the detailed description and/or in the claims, may becombined with steps of the method, when appropriately substituted by acorresponding process.

As used, the singular forms “a,” “an,” and “the” are intended to includethe plural forms as well (i.e. to have the meaning “at least one”),unless expressly stated otherwise. It will be further understood thatthe terms “includes,” “comprises,” “including,” and/or “comprising,”when used in this specification, specify the presence of statedfeatures, integers, steps, operations, elements, and/or components, butdo not preclude the presence or addition of one or more other features,integers, steps, operations, elements, components, and/or groupsthereof. It will also be understood that when an element is referred toas being “connected” or “coupled” to another element, it can be directlyconnected or coupled to the other element but an intervening elementsmay also be present, unless expressly stated otherwise. Furthermore,“connected” or “coupled” as used herein may include wirelessly connectedor coupled. As used herein, the term “and/or” includes any and allcombinations of one or more of the associated listed items. The steps ofany disclosed method is not limited to the exact order stated herein,unless expressly stated otherwise.

It should be appreciated that reference throughout this specification to“one embodiment” or “an embodiment” or “an aspect” or features includedas “may” means that a particular feature, structure or characteristicdescribed in connection with the embodiment is included in at least oneembodiment of the disclosure. Furthermore, the particular features,structures or characteristics may be combined as suitable in one or moreembodiments of the disclosure. The previous description is provided toenable any person skilled in the art to practice the various aspectsdescribed herein. Various modifications to these aspects will be readilyapparent to those skilled in the art, and the generic principles definedherein may be applied to other aspects.

The claims are not intended to be limited to the aspects shown herein,but is to be accorded the full scope consistent with the language of theclaims, wherein reference to an element in the singular is not intendedto mean “one and only one” unless specifically so stated, but rather“one or more.” Unless specifically stated otherwise, the term “some”refers to one or more.

Accordingly, the scope should be judged in terms of the claims thatfollow.

REFERENCES

-   [Schaub; 2008] Arthur Schaub, Digital hearing Aids, Thieme Medical.    Pub., 2008.

The invention claimed is:
 1. A hearing device comprising a forward path for processing an electric signal representing sound, the forward path comprising an input unit for receiving or providing an electric input signal representing sound, a signal processing unit for applying a frequency- and/or level-dependent gain to an input signal of the forward path and providing a processed output signal, and an output transducer for generating stimuli perceivable as sound to a user; the hearing device further comprising a feedback detection unit configured to detect feedback or evaluate a risk of feedback via an acoustic or mechanical or electrical feedback path from said output transducer to said input unit, a loop consisting of said forward path and said feedback path being defined, the loop exhibiting a loop delay D, wherein said feedback detection unit comprises a magnitude and phase analysis unit for repeatedly determining magnitude and phase of said electric input signal or a processed version thereof, and further configured to determine values of loop magnitude, loop phase, and loop phase difference signals, respectively, based thereon and on said loop delay D, where said loop phase difference is the difference between values of the parameter loop phase at a given time instant, m, and a time instant, mD, one feedback loop delay D earlier; a feedback conditions and detection unit configured to check criteria for magnitude and phase feedback condition, respectively, based on said values of loop magnitude, and loop phase difference signals, respectively, and to provide feedback detection signal indicative of feedback or a risk of feedback.
 2. A hearing device according to claim 1 comprising an analysis filter bank for converting said electric input signal to a number of frequency sub-band electric input signals IN(k,m), where k and m are frequency sub-band and time indices, respectively.
 3. A hearing device according to claim 2 wherein the magnitude and phase analysis unit is configured to determine the loop magnitude at time instant m as LpMag(k,m)=Mag(k,m)−Mag(k,m _(D)), where Mag(k,m) is the magnitude value of the electric input signal IN(k,m) at time m, whereas Mag(k,m_(D)) denotes the magnitude of the electric input signal IN(k,m_(D)) one feedback loop delay D earlier.
 4. A hearing device according to claim 2 wherein the magnitude and phase analysis unit is configured to determine the loop phase LpPhase (in radian) at time instant m as LpPhase(k,m)=wrap(Phase(k,m)−Phase(k,m _(D))), where wrap(.) denotes the phase wrapping operator, the loop phase thus having a possible value range of [−π, π], and where Phase(k,m) and Phase (k,m_(D)) are the phase value of the electric input signal, at time instant m and at one feedback loop delay D earlier, respectively.
 5. A hearing device according to claim 3 wherein the magnitude and phase analysis unit is configured to determine a loop magnitude difference LpMagDiff(k,m) at time instant m as LpMagDiff(k,m)=LpMag(k,m)−LpMag(k,m _(D)), where LpMag(k,m) and LpMag(k,m_(D)) are the values of the loop magnitude LpMag at time instant m and at a time instant m_(D), one feedback loop delay D earlier, respectively.
 6. A hearing device according to claim 4 wherein the magnitude and phase analysis unit is configured to determine the loop phase difference LpPhaseDiff(k,m) at time instant m as LpPhaseDiff(k,m)=wrap(LpPhase(k,m)−LpPhase(k,m _(D))), where LpPhase(k,m) and LpPhase(k,m−D) are the values of the loop phase LpPhase at time instant m and at a time instant m_(D), one feedback loop delay D earlier, respectively.
 7. A hearing device according to claim 1 wherein the loop delay D is adaptively estimated during use of the hearing device.
 8. A hearing device according to claim 1 wherein the loop delay D for calculating loop magnitude, loop phase, loop magnitude difference and loop phase difference is a frequency dependent value of loop delay D(k), where k is a frequency sub-band index.
 9. A hearing device according to claim 3 wherein the criterion for the loop magnitude feedback condition is defined as: LpMagDet(k,m)=min(LpMag(k,m), . . . ,LpMag(k,m _(N-D)))>MagThresh, where N is a number of loop delays, m_(N-D) is the time instant N feedback loop delay D earlier, and MagThresh is a loop magnitude threshold value.
 10. A hearing device according to claim 4 wherein the criterion for the loop phase feedback condition is defined as: LpPhaseDet(k,m)=abs(LpPhase(k,m))<PhaseThresh, where PhaseThresh is a threshold value.
 11. A hearing device according to claim 10 wherein a criterion for feedback detection is determined based on a combination of the criteria for loop magnitude and loop phase feedback conditions as FbDet(k,m)=and(LpMagDet(k,m),LpPhaseDet(k,m)).
 12. A hearing device according to claim 9 wherein a criterion for feedback detection is determined based on a combination of criteria for loop magnitude (LpMag) and loop phase difference (LpPhaseDiff) feedback conditions, FbDet(k,m)=and(LpMagDet(k,m),LpPhaseDiffDet(k,m)) where a criterion for the loop phase difference feedback condition is defined as LpPhaseDiffDet(k,m)=abs(LpPhaseDiff(k,m))<PhaseDiffThresh.
 13. A hearing device according to claim 1 wherein the feedback detection unit further comprises a loop transfer function estimation and correction unit receiving as inputs the signals loop magnitude and loop phase, and provides as an output a complex signal representing an estimate of the complex loop transfer function.
 14. A hearing device according to claim 3 wherein a loop magnitude estimate LpMagEst(k,m) is computed as the linear combination of a number P of latest values of loop magnitude LpMag(k,m), ${{{LpMagEst}\left( {k,m} \right)} = {\sum\limits_{p = 0}^{P - 1}{\alpha_{p} \cdot {{LpMag}\left( {k,{m - p}} \right)}}}},$ where α_(p) are non-negative scaling factors, and Σα_(p)=1.
 15. A hearing device according to claim 13 further comprising an action information unit configured to take as inputs the feedback detection signal and the complex loop transfer function estimate from the feedback detection unit and to provide as an output an action information signal.
 16. A hearing device according to claim 15 wherein the action information unit comprises an input control signal configured to activate actions that may influence the feedback detection.
 17. A hearing device according to claim 16 wherein the action information unit is configured to test actions activated via the control signal of the action information unit.
 18. A hearing device according to claim 1 wherein the feedback detection unit comprises different parallel processing units for providing a feedback detection signal FbDet(D_(j)), each being configured to use a different loop delay D_(j), j=1, 2, . . . , N_(D), where N_(D) is the number of different parallel processing units.
 19. A hearing device according to claim 1 wherein the hearing device is or comprises a hearing aid.
 20. A hearing device according to claim 1 wherein the hearing device is or comprises a headset.
 21. A method of detecting feedback in a hearing device, the hearing device comprising a forward path for processing an electric signal representing sound, the forward path comprising an input unit for receiving or providing an electric input signal representing sound, a signal processing unit for applying a frequency- and/or level-dependent gain to an input signal of the forward path and providing a processed output signal, and an output transducer for generating stimuli perceivable as sound to a user, the method comprising detecting feedback or evaluating a risk of feedback via an acoustic or mechanical or electrical feedback path from said output transducer to said input unit, a loop consisting of said forward path and said acoustic or mechanical or electrical feedback path being defined, the loop exhibiting a loop delay D; by repeatedly determining magnitude and phase of said electric input signal or a processed version thereof; and determining values of loop magnitude, loop phase, and loop phase difference signals, respectively, based thereon and on said loop delay D, wherein loop phase difference is the difference between values of the parameter loop phase at a given time instant, m, and a time instant, mD, one feedback loop delay D earlier; checking criteria for magnitude and phase feedback condition, respectively, based on said values of loop magnitude, and loop phase difference signals, respectively, and providing a feedback detection signal indicative of feedback or a risk of feedback.
 22. A data processing system comprising a processor and program code means for causing the processor to perform the steps of the method of claim
 21. 